As a means for forming a multilayer thin film on a substrate, a method which implements the formation of such film by means of a CVD apparatus comprising a plurality of film-forming chambers each having parallel planar electrodes installed therein has been known to date.
Now, a method for forming a silicon nitride film (film A) and an amorphous silicon film (film B) on a substrate by the use of a conventional CVD apparatus comprising a plurality of film-forming chambers each having parallel planar electrodes installed therein will be described below with the aid of a flow chart of FIG. 6.
First, a substrate such as of glass is introduced into a film-forming chamber I for the formation of a film A and set in place on the lower one of parallel planar electrodes (1). The chamber is evacuated to a high degree of vacuum (not higher than 1.times.10.sup.-1 Pa, for example) prior to the formation of the film (2), reaction gases A (monosilane, ammonia, or nitrogen) necessary for the formation of the film A are introduced into the film-forming chamber I and the reaction gases A in the chamber are adjusted to a pressure permitting plasma discharge (3), and an electric power is supplied from a high-frequency power source to the opposed electrodes to initiate plasma discharge and deposition of the thin film A on the substrate (4). After the discharge is continued for a prescribed period of time, the supply of the electric power from the high-frequency power source is cut off to stop the plasma discharge and, at the same time, the supply of the reaction gases A is stopped (5). The film-forming chamber I is evacuated to the high degree of vacuum (not higher than 1.times.10.sup.-1 Pa, for example) mentioned above (6) and the substrate having the thin film A formed thereon is removed from within the evacuated film-forming chamber I without lowering the degree of vacuum (7) and is introduced into a film-forming chamber II for the formation of film B (8). In the film-forming chamber II, the steps of operation performed in the film-forming chamber I are similarly carried out sequentially; the chamber is evacuated prior to the formation of film (9), reaction gases B (monosilane hydrogen) are introduced into the chamber and then adjusted to a stated pressure (10), plasma discharge is continued for a stated period of time (11), the plasma discharge is stopped (by stopping the supply of the reaction gas) (12), the chamber is evacuated after the formation of the film (13), and thereafter the substrate is transferred from the chamber (14).
The evacuation prior to the formation of the film carried out each at the steps (2) and (9) mentioned above is aimed at expelling impure gas, cleaning the surface of the substrate and the empty space for discharge to the fullest possible extent, and preventing entry of impurities into the interface between the substrate and the thin film to be formed thereon and into the thin film itself. To fulfill this aim in the manufacture of a thin film transistor under a prescribed pressure of 100 Pa, for example, the evacuation must be performed until the pressure reaches 1/1000 of the prescribed pressure, namely not more than 10.times.10.sup.-1 Pa. For the evacuation prior to the formation of the film, therefore, a wide-range turbo molecular pump having an ample capacity to evacuate is used. The pump requires a period of about 60 seconds to lower the pressure to this level. In the introduction of the reaction gases A or B into the film-forming chamber and the adjustment of the pressure therein at the step (3) or (10), the reaction gases are exclusively introduced into the film-forming chamber in which a clean atmosphere has been established in consequence of the evacuation prior to the formation of the film and the reaction gases in the chamber are adjusted over a period of about 30 seconds preceding the start of discharge to a pressure necessary for continuation of the discharge and retained at this pressure thence. A flow rate adjusting mechanism is used for the introduction of the gas and an automatic pressure control valve is used for the pressure adjustment. After the interior of the film-forming chamber is adjusted to the prescribed pressure with the reaction gas, the plasma discharge at the step (4) or (11) is started. The film thickness is adjusted by controlling the duration of the plasma discharge before the stop of discharge at the step (5) or (12).
Specifically, for forming the film A (silicon nitride) in a thickness of 400 nm and the film B (amorphous silicon) in a thickness of 300 nm, the plasma discharge must be continued for 120 seconds and 360 seconds respectively. The supply of the reaction gases is stopped at the same time that the discharge is stopped. Then, the evacuation of the chamber after the formation of the film is immediately carried out at the step (6) or (13). This evacuation is performed for the same purpose as the evacuation before the formation of the film mentioned above. Similarly to the evacuation before the formation of the film, the present evacuation is carried out by the use of the wide-range turbo molecular pump over a period of 90 seconds.
For a thin film transistor comprising such film A (silicon nitride) and film B (amorphous silicon) as mentioned above to be continuously formed on a substrate in a vacuum by the plasma CVD technique using parallel planar substrates, it is necessary to use two film-forming chambers. The time required for the formation of the film in this case is 840 seconds in total.
In the formation of film by the use of the conventional CVD apparatus as mentioned above, it is customary to evacuate the film-forming chamber to a high degree of vacuum for the sake of creating a clean atmosphere in the chamber prior to the formation of the film. This evacuation, therefore, not only requires use of the wide-range turbo molecular pump of a fully ample capacity but also entails the problem of necessitating such a duration of time in establishing a prescribed degree of vacuum in the film-forming chamber as may be equal to or more than that which is required before and after the formation of the film.
Further, with the conventional CVD apparatus, the formation of multilayer films requires as many film-forming chambers as the kinds of films involved, which entails the problem of adding to the cost of equipment and enlarging the size of the apparatus and necessitating a proportionately large floor area for the installation of the apparatus.
Besides, since the adjustment of the film thickness depends on the control of the duration of the discharge, a problem ensues that the time of actual start of the discharge is not easily detected accurately and the adjustment of the film thickness is not easily attained with high repeatability.
Another problem further arises that when the electric power from the high-frequency power source is applied, the reflected power from the electrode plate after start of the discharge is large as shown in FIG. 7 and, as a result, the state of initial plasma between the time the reflected power begins to diminish and the time it eventually stabilizes differs from the subsequent state of plasma and the film initially deposited differs in quality from that deposited subsequently. This problem gains in prominence proportionately as the speed of film deposition is increased, namely as the magnitude of the electric power to be applied and the amount of the reaction gases to be supplied are increased. The attempt to confer perfect quality on the film during the initial deposition, therefore, tolerates no large addition to the speed of film deposition and suffers an increase in the time required for the film formation and a decrease in productivity of the film formation.
One object of this invention, therefore, is to provide a method for forming a thin film, which obviates the necessity for such means of evacuation as a wide-range turbo molecular pump which is capable of attaining a high degree of vacuum.
A further object of this invention is to provide a method for forming a thin film, which enables a multilayer film to be continuously formed in a sole film-forming chamber and, therefore, permits miniaturization of an apparatus.
Another object of this invention is to provide a method for forming a thin film, which enables a multilayer film to be produced homogeneously in film thickness and in film quality as well.
Yet another object of this invention is to provide a method for forming a thin film, which requires a short time in the formation of the film and enjoys perfect productivity.
Still another object of this invention is to provide a method for producing a semiconductor device which uses a film formed of a plurality of superposed layers including a semiconductor film and excels in properties.